Linear growth in power law $f(T)$ gravity

We provide for the first time the growth index of linear matter fluctuations of the power law $f(T)\propto (-T{)}^b$ gravity model. We find that the asymptotic form of this particular $f(T)$ model is $\gamma \approx \frac{6}{11-6b}$whichobviouslyextendsthatofthe$\Lambda$CDMmodel,$\gamma_{\Lambda}\approx 6/11$. Finally, we generalize the growth index analysis of $f(T)$ gravity in the case where $\gamma$ is allowed to vary with redshift.